A buried velocity anomaly produces a non-hyperbolic move-out in all the seismic reflection events below that anomaly. This non-hyperbolicity degrades the seismic image after stacking due to the fact that the anomaly imprint cannot be properly described by the stacking velocity function or static corrections. This distortion or deterioration can appear as a defocus or as a break in the seismic image.
Conventional time-imaging techniques such as stacking or migration use stacking velocity to describe the nature of seismic data. Stacking velocity is based on the assumption that all seismic events in the common mid-point domain can be described by hyperbolic functions. In other words, it assumes that the earth's layers in the seismic investigation region have very gentle transformations and contain low relief structures with minimal lateral velocity changes. However, this is hardly ever the case with land datasets that suffer from the complexities of weathered layers and buried velocity anomalies which cause the seismic events to be non-hyperbolic.
There are two major families of solutions which are commonly utilized to compensate for these complexities: statics solutions and solutions. In statics solutions, the main underlying assumption is the vertical ray-path assumption which means that a static shift is sufficient to remove the effect of the velocity anomaly. That is, a single value is applied to all of the different time samples of all the traces that share the same location. However, regardless of the method used to calculate the static solution, it is valid only if the anomaly is close to the earth's surface, that it is laterally smooth and is low in velocity. These assumptions don't hold in the case of complex near-surface or buried velocity anomalies and the static solution cannot accurately resolve the problem of clarifying the data.
Various methods have been proposed for resolving the problem of near-surface anomalies in order to provide more and cleaner data. For example, U.S. Pat. No. 6,151,275 discloses a method for separating seismic data into a first set of seismic data identified as upgoing seismic data, and a second set of seismic data identified as downgoing seismic data. The first and second sets of seismic data are redatumed to a target horizon to provide two sets of seismic data; the new sets are combined to create an image. Thus, this method tries to first separate the primaries from the multiples and then stacks them together after adjusting the datum difference between the two sets. The two sets of upgoing and downgoing data correspond, respectively to the primaries and first order multiples. Since no dataset is produced, it will be understood that the ultimate goal of the process described in this patent is to enhance the produced image by summing the primaries and multiples in the same image point without returning to the original surface of the data.
Redatuming solutions are much more accurate because they resolve the problem by calculating corrections that are dynamic in time, as well as in offset. One limitation of most redatuming algorithms is that they require the use of knowledge of the velocity-depth model of the near-surface which is very difficult to obtain in land datasets. The exception is common focus point (CFP) redatuming which requires only knowledge of the one-way traveltime operators to perform the redatuming. Traveltime operators are the one-way time that it takes the wave-field to travel from source/receiver point to a reflection point of a target horizon to which the data will be redatumed. However, CFP redatuming also has certain limitations, including the following:                a. the data above the target horizon is degraded because the redatuming process shifts all the anomalies to those shallower horizons;        b. the new dataset after redatuming has an unknown acquisition reference in location/depth that is the target horizon; and        c. the new dataset after redatuming is different than the input dataset in reference time, as well as in the move-out behavior of the seismic events.        
CFP-based redatuming is performed using one-way traveltime operators from the surface to a target horizon. The redatuming process produces a dataset which simulates a survey as though the sources and receivers were positioned at the chosen target horizon. Thus, if the target horizon is below the buried velocity anomalies, the redatuming process will shift the chosen imprints of the anomalies from below the target horizon to above the target horizon, which is referred to as the anti-causal part of the resulting data. The traveltime operators used in CFP redatuming, which are denominated true traveltime operators, exactly describe the target horizon in the one-way time domain. This means that if the data is converted to one-way time, e.g., by creating CFP gathers, or if the operators are converted to two-way time, e.g., by using Fermat's principle, a match should be obtained.
In order to illustrate the limitations of the corrective measures of the prior art methods, reference will be made to the simplified schematic illustration of FIG. 1 where a buried anomaly is positioned at “A”.
It is clear that for a point source at (x, z)=(0, h) and receivers at the surface, the effect of the buried anomaly will appear on the receiver from x=xmin to x=xmax, where:xmin=x0*h/(h−h0) andxmax=x1*h/(h−h0)If “d” is the total distance where the effect is measured:d=xmax−xmin=h*(x1−x0)/(h−h0)However, if h0 is very small or if h is very large, i.e., h>>h0, then:xmin=x0 and xmax=x1
It is noted that xmin, xmax and d are dependent on h. This means that the effect of the buried velocity anomaly will vary in offset and value as a function of time. A static solution therefore cannot resolve this effect, even if trim statics were used, because of the dynamic nature of the problem.
In addition, it will be understood from the above equations that the only conditions where the effect is not dynamic are when the anomaly is very shallow or the horizon of interest is very deep. Static corrections will provide a satisfactory resolution for shallow anomalies; however, as is well known, although static corrections might resolve the problem for the very deep horizon, it is at the cost of the horizons closer to the anomaly. Another problem that arises with buried velocity anomalies is the possibility of having horizons above the anomaly, in which case, it is possible to resolve the problem for the deeper horizons, but the effect of the anomaly will be imposed on the horizons above it.
Referring to FIG. 2, a simple layered model is illustrated that is 5000 m wide with a buried velocity anomaly at x=2500 m. FIG. 3 depicts the reflected events in different CMP gathers which were calculated by ray tracing to illustrate the variable effect of the buried anomaly of FIG. 2. The common-mid-point (CMP) gathers were taken at x=2300 m, 2400 m and 2500 m.
The method of removing the effect of the buried velocity anomaly by CFP-based redatuming utilizes one-way traveltime operators from the surface to a selected target horizon. Referring to FIG. 4, there is depicted a simple layered model with a buried anomaly and its CMP gather. The left chart schematically depicts the sources (stars) and receivers (triangles) at the surface (h0) and a buried velocity anomaly (A) between horizons h1 and h2. The graph to the right depicts the offset for various CMP gathers. The redatuming process produces a dataset which simulates a survey as if sources and receivers were positioned at that target horizon.
The new dataset will have two parts: a causal part and an anti-causal part. The causal part shows the reflection coming from below the target horizon and the anti-causal part shows the reflection coming from above the target horizon. In order to remove the effect of a velocity anomaly from the deeper horizon, redatuming to any horizon that is below the anomaly is performed.
Referring now to FIG. 5, there is depicted a CMP gather after redatuming with true traveltime operators. Although the redatuming process successfully removed the anomaly from horizons 3 and 4 in FIG. 5, the redatuming imposed it on the horizons 0 and 1. Another problem is that this new dataset differs from the input data in two important ways. First, the new dataset has the target horizon flat at zero, which is considered to be undesirable by interpreters of the resultant image because they are used to looking at data from a smoothed surface. Second, the new stacking velocity is very different than the original stacking velocity, which means that the velocity analysis should be repeated from scratch.
It is therefore an object of the present invention to provide a process for the correction of seismic data to minimize the effect of distortions caused by a near-surface anomaly that produces a non-hyperbolic move-out in the seismic reflection below that anomaly.